This invention relates to a target of an image pickup tube for television, and more particularly to a target of image pickup tube capable of reducing the after-image effect when operated at a high temperature.
Amorphous selenium (Se) has a photoconductivity and generally also has a p-type conductivity, forming a rectifying contact with an n-type conductive material. Thus, a photodiode type target of an image pickup tube can be made from the amorphous Se on the basis of these characteristics. However, amorphous Se is insensitive to the long wavelength of light and it has been a practice to add tellurium (Te) to a region of a Se layer to improve the sensitivity to the long wavelength of light (U.S. Pat. No. 3,890,525 and U.S. Pat. No. 4,040,985).
Furthermore, it has been also proposed to add a specific fluoride to a region of the p-type photoconductive layer to improve the responsiveness when an incident light of high intensity is cut off (U.S. Pat. No. 4,330,733).
FIG. 1 shows one example of a target structure according to the prior art, wherein numeral 1 is a transparent substrate, 2 a transparent conductive film, 3 a p-type photoconductive layer made from Se-As-Te, 4 a p-type photoconductive layer made from Se-As, and 5 a landing layer of the scanning electron beam made from porous Sb.sub.2 S.sub.3. Te is a component for enhancing the sensitivity to red light, as described above, and arsenic (As) is a component for increasing the viscosity of an amorphous film composed mainly of Se and enhancing the thermal stability. With this structure the target can act as a photodiode type to block the injection of holes and scanning electrons and thus can have such imaging characteristics as less dark current and less lag.
The target of an image pickup tube according to the prior art can have good imaging characteristics under the normal operating conditions, but still has such a drawback as an increased after-image when operated at a high temperature, because no thorough consideration is given to a higher temperature during the operation of image pickup tubes.